1. Technical Field of the Invention
This invention relates to an apparatus and method for passive testing of the electrostatic discharge systems (ESD) installed on all types of aircraft.
2. Background of the Invention
Air travel and cargo transport has been steadily increasing over the last few decades, resulting in increased number of flights and flight locations. The flights require around the clock transportation of persons and cargo and in all types of weather. With the increasing number of planes in the air at a given time, there is a growing need to keep planes in the air for longer periods and with less maintenance times.
Since deregulation, the commercial air travel industry has experienced dramatic fluctuations in the carriers. Various carriers have come and gone while others have experienced serious economic difficulty. An unfortunate side effect of the economics of air transportation is that routine maintenance schedules and number of experienced maintenance personnel have both been lowered.
An undesirable aspect of aircraft flight is that the aircraft itself often suffers from an accumulation of electrostatic charges through various effects including atmospheric conditions and the motion of the aircraft or parts thereof through the atmosphere. The accumulation of the electrostatic charges tends to build up potentials than can become dangerous and can cause radio frequency interference, electrical shock to persons connected with the aircraft, equipment failures and unwanted ignition of fuel and armament.
For example, there have been some highly publicized plane explosions linked to the build-up of the static charges, particularly in the vicinity of the fuel tanks of the aircraft. There is growing speculation that other plane crashes may be indirectly linked to static electrical problems and failure of aircraft electronics and vital flight systems.
In addition, static charge accumulation also interferes with the radio communication operations or other amplitude modulated equipment that is described in the prior art. Such communication interference creates a particularly hazardous situation during flight operations that include landing of the aircraft since it is necessary that the aircraft be in constant communication with ground control and/or other aircraft during flight operations.
The atmosphere naturally carries a positive charge of approximately 100,000 Volt at a typical flying height of 10,000 meters altitude. The atmospheric charge is associated with the electrical field existing between the Earth and the ionosphere. The low humidity environmental condition at a typical flying altitude is a perfect environment for buildup of large electrostatic charges. It is reasonable to assume that an aircraft can acquire a charge of one million Volts or more, which it shares with the passengers inside the aircraft. The positive charge that is induced by friction from an airplane flying through the atmosphere compounds the positively charged airplane and due to the size of larger planes, an aircraft can acquire a charge of greater than one million Volts.
During landing, especially in dry conditions, sparks can be released to the ground as soon as an aircraft touches down thereby discharging the high potential difference. Larger planes would inevitably have a greater potential, and to facilitate the grounding of the aircraft, metal-rods are fitted to the tips of the aircraft wings in an effort to discharge the aircraft. The static discharge antennas are normally located on wing tips, rudder and elevator.
There is an obvious hazardous condition if the potential is not discharged from the airplane for the personnel that may come into contact or close proximity to the plane. There are detailed guidelines for refueling that require a grounding strap to discharge any accumulated charge promulgated by various federal and state agencies. However, accidents do occur whether by faulty grounding systems or operator error. Thus there is a general desire to ensure that accumulated charges are dissipated.
The static discharge antennas also serve as lightening rods so that when struck by lightening the electricity finds an exit location instead of exiting from other flight control mechanisms.
In addition to the electrostatic discharging rods, most commercial aircraft also use grounding straps to dissipate any charge that did reside. Upon landing, the aircraft employs grounding straps that strike the ground upon landing and discharge accumulated charge from the aircraft skin. While not able to mitigate the static buildup in flight, these straps are supposed to ensure that any charge that did accumulate is properly dissipated to ground.
The grounding straps are subject to maintenance testing both visual and ground based testing. The straps may have broken conductors or connectors that prevent proper electrical connectivity. The testing procedures are costly as the plane is taken out of service and requires skilled maintenance personnel run the testing procedures.
There are some prior art devices for measuring the strength of the electrostatic field between the aircraft and the surrounding environment in order to determine whether or not the electrostatic potential of the aircraft is at a dangerous level. And, there are various electrostatic discharge systems designed to reduce or eliminate charge build up.
Aircraft in flight acquire static electrical charges in several ways. The principal charging is called precipitation charging, and includes charging due to rain, snow, ice crystals and dust. The precipitation charging imparts a net electrical charge on an aircraft and, if allowed to accumulate, can produce corona discharge and severe electrical noise interference in the airborne navigation and communication systems aboard the aircraft. Discharge currents of several milliamperes and airplane voltage of over 200,000 volts have been measured during precipitation charging conditions. A general description of aircraft electrostatic charge is described in the article by the US Army Research Laboratory entitled Electric Field Microsensors: Concept and Model Validation as presented in the Military Magnetic and Electric Field Workshop Oct. 19, 2000.
In U.S. Pat. No. 4,825,149, there is an electric field sensor with a disk electrode coupled to a circuit having a resistor parallel to a capacitor. The output is measured by an oscilloscope, and is described in terms of detecting an electromagnetic pulse accompanying a nuclear detonation.
Published U.S. patent application U.S. 2002/0082769 ('769) discloses an airport auditing and management architecture. This described system encompasses various acoustic sensors and cameras that monitor aircraft activities and coordinate the activities in a central processing location. While not particularly relevant to the sensing device of the present invention, the '769 system illustrates how the sensor data could be implemented into an existing architecture and interact with the various related organizations to monitor electrostatic discharge systems and provide proper maintenance.
There are various electrostatic discharge systems discussed in the prior art, including the helicopter system of U.S. Pat. No. 3,260,893. The operational characteristics describing triboelectric charging is detailed along with the discharging system employed. There are also known devices on airplane wings to discharge static electricity as it is generated, for example, by the friction of water droplets as the plane flies through a cloud. These devices attempt to reduce the charge to levels that might cause radio interference and to prevent explosions such as the Apollo 13 disaster. Faulty electrostatic discharge systems may be responsible for other aircraft crashes whether directly by sparking near a fuel tank or indirectly by damaging vital electronics. And, a faulty discharge system may also lead to more damage from a lightening strike resulting in disaster.
There are various descriptions for detection of weather conditions conducive to lightening, including U.S. Pat. Nos. 5,828,334 and 3,790,884 which generally describe a plurality of antennas that sense changes in the electrostatic field between antennas. A somewhat related invention is described in U.S. Pat. No. 4,975,686 that describes an electrostatic field detection system for persons and furniture in the vicinity of computer electronics.
Electrostatic discharge (ESD) devices are well known, and used with aircraft structures so as to provide for the elimination of static electrical charges accumulated thereon. Such static discharge elements help protect radio systems from precipitation static noise. Prior art ESD designs include trailing discharge elements and tip dischargers that extend beyond the aerodynamic airfoil surfaces in the form of rods. These static discharge elements are constructed of rod-like configurations that extend from a surface portion of an airplane. However, such protrusions increase drag and furthermore are subject to breakage as a result of the additional stress during flight and also by the inadvertent mishandling of the ground personnel during a routine maintenance operation. The rods are also vulnerable to damage from lightning, hail, vibration, bird strikes, and mechanical damage during ground handling. As a consequence, aircraft departure is sometimes delayed if the discharge elements are missing or damaged. The key discharge units are usually the outboard trailing and tip dischargers that are in the highest electrical fields during static charging conditions.
Other static dischargers include using knitted wire mesh material onto a glass fiber composite or electrically nonconductive structural panel, such as a flight control surface of an aircraft, for forming an electrically conductive exterior surface that controls the accumulation of electrostatic charge and protects the underlying panel structure from damage by lightning strikes.
However all the various ESD systems require inspection, maintenance, and repair in order to ensure the safety of the aircraft. The Federal Aviation Agency (FAA) is the US Government agency primarily responsible for managing commercial air travel. They establish the guidelines and requirements and are the responsible for enforcing those requirements. ESD systems are required on all commercial aircraft. These devices are installed and regularly inspected by the skilled ground maintenance personnel and records are supposed to be accurately maintained.
One device for measuring the electrostatic field is called a field mill type. A field mill is a device for measuring electrostatic field strength on the basis of charges induced on a sensor electrode within the field mill that is alternately covered and exposed to the electrostatic field sought to be measured, wherein the alternate covering can be accomplished either electrically or mechanically. The induced charge on the sensor electrode of the field mill is proportional to the external field impinging on the sensor electrode. With appropriate phase convergence, both the strength and polarity of the electrostatic field can be determined.
However, the testing is only done periodically. There is also no current means available for testing the ESD systems in flight. And, there is no way of knowing whether a tested unit will perform adequately until the next scheduled testing/maintenance. The typical maintenance inspection requires experienced field service personnel to use specific instrumentation to check the equipment.
Therefore, it is well known that the buildup of static electricity poses in-flight and ground based hazards. The airborne static discharge system and landing grounding techniques are subject to malfunctions. The routine maintenance schedule is helpful, but may not locate a faulty system until the next scheduled maintenance. What is needed is a system that can passively detect the functionality of aircraft ESD systems and provide low cost diagnostic system for airborne static discharge and grounding systems. Such a system should be inexpensive to install and should be able to monitor aircraft departing or returning in order to schedule ESD maintenance. In order to be easily implemented, such a system should be simple and easy to operate and install. If such a system were available, it would result in cost-savings in relation to maintenance inspections, and, non-implementation could result in liability issues. However the greatest need is for safer air travel as the hazards from the build up of electrostatic charge are extreme.